1. Field of the Invention
The present invention relates to an outboard motor including an engine, an engine cover forming an engine compartment for holding the engine therein, a top cover covering the engine cover from above, a ventilation system for ventilating the engine compartment, and so on.
2. Description of the Related Art
A known outboard motor disclosed in, for example, JP 2002-240785A includes an internal combustion engine, an engine cover forming an engine compartment for holding the internal combustion engine therein, and a top cover covering the engine cover from above, joined to the engine cover and provided with a carrying grip.
The carrying grip formed in the top cover of the outboard motor is gripped when the top cover and the engine cover are handled together for mounting and dismounting. Therefore, connecting parts for connecting the top cover and the engine cover are protrusions formed on the top cover or the engine cover and having a necessary rigidity.
When the connecting parts are formed on the small top cover of small size near its periphery where the thickness of the space between the top cover and the engine cover is small, the connecting parts are of low height, so that a necessary rigidity can be ensured for the connecting parts, and a necessary rigidity can be ensured for the top cover.
A space defined by the engine cover and the top cover is used as an air passage, such as an intake passage through which intake combustion air flows or a ventilation passage through which ventilation air for ventilating the engine compartment flows. The air passage has an air inlet through which air is taken in. The air inlet is formed in a duct extending upward in the air passage to suppress water flow such as sea water spray or rainwater drops, through the air inlet into the air passage.
In some cases, connecting parts need to be formed in parts of the top cover which are spaced widely apart from the engine cover when the top cover is of so large a size as to cover all or a major part of the top wall of the engine cover from above.
Connecting parts formed in such parts of the top cover spaced widely apart from the engine cover are inevitably high and have a low rigidity. Therefore, there is a limit to the distance between the engine cover and the parts in which the connecting parts are formed. If the height of the engine cover is increased to reduce the thickness of the space between the engine cover and the top cover from the viewpoint of ensuring a necessary rigidity for the connecting parts, a large mold is required for manufacturing the engine cover and hence the manufacturing cost increases. When the duct having the air inlet is formed in the engine cover, the manufacturing cost increases because the engine cover has a complicated shape.
Further, it is desirable that the space between the engine cover and the top cover imposes less restrictions on the arrangement of the connecting parts to enhance the rigidity of the engine cover and the top cover or to distribute a load placed through the top cover on the engine cover when the engine cover and the top cover are mounted and dismounted together by holding the top cover by the carrying grip.
Water is liable to flow into intake air for combustion when the intake air flows from the air-intake space defined by the engine cover and the top cover through an intake system into the combustion chambers of the internal combustion engine. A known outboard motor disclosed in, for example, JP 2006-151242A is provided with a baffle for preventing water from flowing into the combustion chambers.
When an intake system disposed in an engine compartment of an outboard motor opens into an air-intake space extending outside the engine compartment, the temperature of intake air that flows from the air-intake space into the intake system is lower than that of intake air that flows from the engine compartment into the intake system after the same has been heated in the engine compartment by heat radiated from the internal combustion engine. Such intake air of low temperature enhances the volumetric efficiency and output performance of the internal combustion engine.
However, since the intake system opens into the air-intake space extending outside the engine compartment, intake pulsation caused by the internal combustion engine is transmitted through the intake system to the air-intake space. Since the air-intake space is defined by the top cover and the engine cover, the engine cover is vibrated by the intake pulsation transmitted to the air-intake space to generate noise.
Known outboard motors disclosed in, for example, JP 5-286490A and JP 2007-38989A include a ventilation system forming a discharge passage through which air in an engine compartment is discharged to the outside and which are provided with an intake passage having an air inlet opening into a space outside the engine compartment.
In an internal combustion engine provided with an intake system having an intake passage forming structure which forms an intake passage having an air inlet opening outside an engine compartment and which is disposed in an engine compartment, air outside the engine compartment (hereinafter referred to as “outside air”), namely, intake air, flows directly into the intake passage. Therefore, the temperature of intake air that flows directly into the intake passage is lower than that of intake air that flows from the engine compartment into the intake passage after the same has been heated in the engine compartment by heat radiated from the internal combustion engine. Such intake air of low temperature enhances the volumetric efficiency of the internal combustion engine.
When a ventilation system is disposed in the engine compartment, the temperature of air flowing from the engine compartment to the outside of the engine compartment through a discharge passage is comparatively high. It is desirable to avoid heating of intake air for combustion by the air flowing through the discharge passage from a viewpoint of preventing the volumetric efficiency from being reduced.
If the discharge passage forming structure forming the discharge passage having an air inlet opening in the engine compartment and a ventilation outlet opening outside the engine compartment is disposed in a combustion air intake space extending outside the engine compartment, it is possible that intake air is heated by the heat of air flowing through the discharge passage through the discharge passage forming structure, thus causing reduction of volumetric efficiency.
In a power unit including an intake passage forming structure disposed in an engine compartment, it is desirable for the enhancement of volumetric efficiency to avoid exposing the intake passage forming structure to the high-temperature air in the engine compartment to the utmost by preferentially discharging air of a comparatively high temperature in the air in the engine compartment. A ventilation system needs to cool the engine body and engine accessories attached to the engine body, such as a generator, by ventilation air taken in from a space outside the engine compartment. When the discharge passage of the ventilation system and the intake passage of the intake system are formed in the engine compartment, it is desirable to suppress heating of air in the intake passage by the heat of the air flowing through the discharge passage.
A known outboard motor, having an engine cover forming an engine compartment for holding an internal combustion engine therein, disclosed in, for example, JP 4-166496A includes first and second air passage forming structures which form an air passage extending between a space outside the engine compartment and a space in the engine compartment and which are joined together with a sealing member held therebetween.
The sealing member made of rubber is exposed to the air passage and is compressed between the first and second air passage forming structures. The sealing member needs to have a rigidity to resist deformation that may be caused by pressure exerted thereon by the air flowing through the air passage.
If, while the sealing member has such a rigidity, a strong force is required for holding the sealing member in a predetermined shape between the first and second air passage forming structures when the first and second air passage forming structures are connected together during the assembly of the outboard motor and, consequently, the efficiency of work is lowered for connecting the first and second air passage forming structures.
When at least either of the first and second air passage forming structures is a part of a cover and the cover is used for connecting the first and second air passage forming structures with the sealing member held therebetween, the efficiency of work for connecting the first and second air passage forming structures is lowered still further.